These are some of the approximately 1,000 birds that slammed into McCormick Place in Chicago on Oct. 5, 2023. That was a night and morning when especially high numbers of birds were migrating along Lake Michigan. The building’s windows have since been treated with a film that makes them visible to birds. Very few birds have hit the windows since then.
An ornithologist spent four decades tracking 40,000 bird deaths at a single building. His records paved the way to better scientific and public understanding.
In 1978, David Willard was working at the Field Museum in Chicago when he found out that birds kept slamming into the city’s lakeside convention center.
He began checking around McCormick Place for dead birds, and soon realized the extent of the problem. Over the next 40 years, Willard and his colleagues found 40,000 of them at that site.
“It’s not a building that I think anybody – when it was built – would predict would be a major bird killer,” said Willard, noting that it isn’t a skyscraper. “But it is right on the lake and has a massive amount of glass.”
Migrating birds tended to strike the lakeside building overnight or in the early morning each spring and fall. As they flew along Lake Michigan, the building’s lights drew them in.
The Field Museum’s careful documentation of the situation decade after decade increased scientific understanding and public concern. It helped pave the way for treating McCormick’s windows with a film that makes them visible to birds. That recent change may spare many thousands of the creatures in coming decades.
“In the past, we would watch them fly straight into the window and drop,” said Willard, now retired from his position as manager of the museum’s bird collection. But after the film was installed last summer, “we’d watch them fly in and veer” safely away.
Cities in the middle of the continent, such as Chicago, Kansas City and Dallas, take a heavy toll on migrating birds. For birds, these population centers pose endless hazards right along one of the most important migration corridors on the planet. 
But the more scientists learn, the clearer it becomes that options exist to make buildings safer. Those options include reducing light pollution and helping birds see windows as solid surfaces.
Andrew Farnsworth, a visiting scientist at the Cornell Lab of Ornithology, said the efficacy of these options makes solving bird collisions less daunting than many other environmental calamities.
“This one is a pretty easy one,” he said, “in the sense that there are very specific kinds of actions that lead to conservation successes.”
The role of artificial light
The stakes are high for making buildings safer. Scientists believe building collisions in the U.S. alone kill 1 billion birds annually.
“It’s almost unfathomable,” Farnsworth said. “That’s like a bird every 30 seconds.”
Fatalities rise during migration. Most species fly under the cover of darkness. In recent decades, it has become clear that artificial light confuses them.
When a group of scientists crunched the Field Museum’s detailed records of the collisions at McCormick Place, it revealed a telling pattern.
When all of the center’s windows were lit, bird deaths rose tenfold compared to when just half of the windows were lit, Willard and his co-authors wrote in a 2021 study published in the Proceedings of the National Academy of Sciences.
In other words, buildings can be made safer in cases where it’s possible to cut their brightness by closing curtains or turning off unneeded indoor bulbs and facade lighting.
Light pollution poses particular dangers wherever large numbers of birds must pass.
Seven of the U.S.’ top 10 riskiest cities for migrating birds each spring are in the middle of the country, according to a 2019 study published in Frontiers in Ecology and the Environment.
From south to north, these are San Antonio, Houston, Dallas, St. Louis, Kansas City, Chicago and Minneapolis.
The explanation? These cities are situated along a vital migration corridor, said Farnsworth, a co-author on this study and the one about McCormick.
“There is an incredible tailwind scenario because of the way the Gulf of Mexico is positioned,” he said, “and the Rocky Mountains creating these low-level jet streams that are basically like southerly winds blowing from the south that are super supportive.”
On some heavy migration nights, hundreds of millions of birds take advantage of the mid-continent’s winds to move along this route. This journey has served their kind well for millennia, but today they face cityscapes that pose new hazards they didn’t evolve to handle.
Daytime versus nighttime collisions
The North American bird population has shrunk by a quarter over the past half century, scientists estimate. Building collisions are a key reason. (Others include habitat loss, climate change and the decline of insects, for example.)
These fatalities can happen at night or during the day.
Light pollution is the key driver of nighttime deaths, luring birds toward human-made structures.
Daytime collisions often happen because birds see sky or trees reflected in windows or because they spot greenery on the other side of the glass. They then fly toward the glass, unaware that it is a solid surface.
In recent years, a number of high-profile tragedies have drawn attention to the dangers of buildings and glass for birds.
On May 4, 2017, about 400 migrating birds slammed into a single office building in Galveston, Texas. The company that occupied it decided to turn off any unneeded lights, such as facade lighting, during migration season.
A growing Lights Out Texas movement calls on building owners and homeowners to turn off unneeded lights overnight – 11 p.m. to 6 a.m. – during peak migration. Two dozen Texas cities and counties have pledged their help.
Lights Out programs exist in other states, too, and in Canada. A fledgling project called Lights Out Heartland is attracting partners in Missouri, Kansas and neighboring states, including Johnson County Community College, Washington University, the St. Louis Public Library and the Kansas City Zoo.
On Oct. 5, 2023, about 1,000 birds hit McCormick Place. The incident – the worst that Willard has ever recorded at McCormick – prompted the metropolitan agency that operates the building to install the dotted film that now makes the windows visible to birds.
The film cost $1.2 million. Two crews alternated on morning and evening shifts for three months to apply the film to two football fields’ worth of windows.
Last fall marked the first migration season with the treated windows. Field Museum folks typically find hundreds of birds each fall. Last fall, they found just 18 had slammed into the treated windows.
Next they will study whether the trend – a 95% decrease in fatalities – holds up during this spring’s migration.
Separately, some scientists, conservation groups and bird advocates have signed an open letter to the U.S. Green Building Council, asking it to incorporate bird safety into LEED certification – a widely used rating system for designing green buildings.
Farnsworth, at Cornell, said people don’t need to own office buildings or even live in the middle of a bustling downtown to stop birds from hitting human-made structures.
“The notion that the problem is just in cities is wrong,” he said. “Your kitchen window, your living room window, your glass door – whatever it is. When you walk outside and you find a dead bird beneath glass, you can address that.”
That could mean turning off unneeded lights outside your house or drawing the kitchen curtains closed at night. It could also mean making glass visible to birds.
He recommended visiting the American Bird Conservancy website to learn more. Some of the options include window screens, subtle vertical cords, translucent tape stripes and the kind of dotted film used at McCormick.
